1. Field of the Invention
The present invention relates generally to a method and apparatus for removing undesired material from internal spaces. More particularly, this invention relates to a method and apparatus for removing undesired material formed in internal spaces of a part from elevated temperature reactions between impurities in air passing through these internal spaces. Specifically, this invention relates to removing undesired material from the internal spaces of air cooled gas turbine engine blades.
2. Description of the Prior Art
Many mechanical parts which are utilized in the aircraft industry are subjected to extreme conditions in regard to temperature and speed. The mechanical parts of a modern gas turbine engine are designed to obtain maximum efficiency from the engine.
A typical gas turbine engine contains a series of turbine rotors containing blades. For example, a typical turbine assembly for a dual compressor turbofan engine contains four stages of turbine rotors and blades.
Turbines are subjected to both high speeds and high temperatures. High speeds result in high centrifugal forces and, because of high temperatures, turbines must operate close to temperature limits which, if exceeded, lower the strength of the construction materials used in the turbines. Because the limiting factor in most turbine engine design is the maximum temperature that can be tolerated at the turbine inlet, engine design engineers have used every device at their command to increase the allowable inlet temperature. Typically, turbine inlet temperatures of advanced engines exceed the thermal limits of the turbine blades and vanes used to extract useful work from the working gas. Designers have overcome this limitation by recognizing that excess air flows in the compressor section of gas turbines. A small portion of the relatively cool compressor air is directed around the combustion chamber into the turbine components by means of ducts and channels. This air extracts heat from the working turbine gases and keeps the components at safe operating temperatures.
On many large engines, turbine inlet guide vanes and the rotor blades are cooled by conducting compressor "bleed air" through passages inside the engine to the turbine area where the air, which acts as a coolant, is led to longitudinal holes, tubes, passages or cavities in the first stage and second stage vanes and blades.
In the simplest cooling method, this so called "bleed air" is directed through a cavity or longitudinal holes machined or cast into the turbine part. Air enters at the blade root and exits at the tip. A more complicated method of component cooling consists of incorporating serpentine (winding) passages into the component. These passages increase the surface area of the part in contact with the cooling gas, thereby allows the cooling gas to extract more heat. Air enters at the base of the blade, travels through the passages, and returns to exit at the base. A further embodiment of the serpentine cooling passage scheme incorporates exit holes at tip, leading edge, and trailing edge locations, as determined by design calculation and tests, to force cooler air over the external part surface. Although the "bleed air" coming from the compressor may be hot, it is cool in relation to the temperature at the turbine inlet. The air, therefore, serves to cool the vanes and blades, thus permitting the gases coming from the burner section of the engine to enter the turbine at a higher temperature than would otherwise be permissible. Generally, cooling is necessary only in the area of the turbine inlet because enough energy is extracted from the exhaust gases by the first or second stage blades of the turbine to reduce the temperature of the gases to a tolerable level.
All cooling schemes suffer a loss of efficiency if the cooling air contains small particulate matter which becomes trapped in the cooling passages. This matter is a complex aluminosilicate that is formed from elevated temperature reactions between impurities and the bypass cooling air as it is swept into the leading and trailing edge corners of these hollow blades. Under extreme conditions, this undesired material may build up to completely block the air flow and this blockage can result in destruction of the blade and extensive engine damage. More typically, the contaminating particulates accumulate in dead zones, or pockets, within the cooling passage partially blocking flow. If this contaminating matter is not removed at time of overhaul, it can react with the base metal of the blade and cause catastrophic loss of strength.
Current procedures for cleaning the internal passages of the hollow blades involve immersing the blades in a 45% solution of potassium hydroxide at400.degree. Fahrenheit under 200 psig for 24 hours with agitation. The blades are then flushed under pressure, both internally and externally, and boiled in distilled water. The blades are then checked for residual salts with a conductivity water check and air dried. Even though this procedure uses numerous flushes to eliminate the potassium hydroxide and a conductivity check, this procedure still leaves harmful residue in the passages which can damage the blade and lead to engine failure. Thus, there is a need in the airplane industry to find a noncorrosive method of removing and clearing out the undesired material which is deposited in the internal spaces of air cooled turbine engine blades while these blades are in use in the engine.
The present invention satisfies this need by providing an improved method and apparatus for performing this method, which removes the undesired material from the internal spaces, such as, passages or cavities of the turbine blades and does not leave a harmful residue. Thus, the present invention provides a method for cleaning internal spaces of air cooled blades, which does not expose the blades to harsh corrosive conditions which can contribute to blade and ultimately engine failure.